Under the Gauss-Markov Theorem When Assumptions SR1 - SR5 Are $\beta$

Multiple Choice

Under the Gauss-Markov Theorem when assumptions SR1 - SR5 are met,what estimators of $\beta$ ₁ and $\beta$ ₂ may have smaller variances than b₁ and b₂?

A) none
B) a non-linear estimator
C) a normally distributed estimator
D) an estimator derived from economic theory